Conventionally, when image display devices such as a projector are used, an image processing method capable of achieving appropriate color reproduction is employed in order to adapt to a change in the brightness of external illumination as one kind of changes in the use environment, as disclosed in Patent Document 1, for example. More specifically, a brightness value of reflected light from a screen due to the external illumination is measured using an optical sensor, and a video signal is subjected to brightness correction/color correction, so that appropriate color reproduction can be achieved.    Patent Document 1: Japanese Patent No. 3719498